Directional couplers are often used together with power amplifiers in order to identify rapidly and safely whether an error matching is present or not. If the power of the forward and returning high-frequency signal can be registered, the power amplifier can be automatically switched off before any damage occurs when a guide value for the returning high-frequency signal is exceeded. The broadband properties of a directional coupler of this kind are particularly necessary for interference-prevention tests, in order to investigate the functionality of the device under test within a broad frequency range. Accordingly, dependent upon the frequency range and the test standard, different antennas and setups are used for coupling interference signals into the device under test. Broken cables, error plug connections or faulty antennas are only some of the possibilities which can lead to error matching. Dependent upon cable attenuations and the antenna characteristic, the necessary output power of the power amplifier varies significantly over the frequency range under test.
A generic arrangement of the directional coupler is already known from U.S. Pat. No. 6,066,994. The directional coupler is structured in a symmetrical manner, wherein the outer conductor of the coaxial line is split at two places and connected in each case via a shunt resistor to the outer conductor of the coaxial line at the input and output. A ferrite core ensures that both ends of the outer conductor are insulated from one another for low frequencies. A measured voltage proportional to the inner-conductor current decreases across the shunt resistor, which is polarised differently dependent upon the direction of the power flow. A capacitive voltage splitter, which is connected to the inner conductor, generates a measured voltage proportional to the inner-conductor voltage. This measured voltage is applied to the gate of a field-effect transistor (FET). Source and drain of the FET are connected via several resistors to the direct voltage source. The measured voltage proportional to the internal-conductor voltage and de-coupled at the source connection is added to or subtracted from the measured voltage proportional to the inner-conductor current decreasing across a shunt resistor, a capacitor, a resistor and a coaxial cable. In the case of matching, both voltages are added in one branch, but they are subtracted in the other branch to form a zero voltage. In the case of an error matching, the voltage difference is not zero. In the case of an infinite VSWR (Voltage Standing Wave Ratio; standing-wave ratio), both voltages are of the same magnitude.
The disadvantage with the arrangement of U.S. Pat. No. 6,066,994 is a complicated structure required in order to integrate all other components into the overall system. The necessary resistors are principally designed as wired resistors. The bending of the connecting arms of the resistors and respectively the coaxial cables, and also the soldering process itself, must be carried out manually. This means that each directional coupler provides slightly different properties. Furthermore, the capacitive voltage splitter has proved disadvantageous. To ensure that the directional coupler can operate at all in the low frequency range, this capacitive voltage splitter must hardly be loaded. Accordingly, the use of an FET as an impedance converter is indispensable. Moreover, two further voltage sources are necessary for the FET itself, which further increases the already high wiring costs and makes an external voltage supply additionally necessary.